Modulation codes have been employed within magnetic recording channels in order to achieve a more even distribution of magnetic flux transitions within a data track in view of data patterns of particular user information being encoded and recorded. In particular, run length limited (RLL) modulation codes have been employed within partial response signaling, maximum likelihood detection (PRML) data recording and playback channels, decision feedback equalization (DFE) channels, and fixed delay tree search (FDTS) channels.
Partial response channels of interest for magnetic data storage devices include a (1-D) dicode channel, a PR4 (1-D.sup.2) channel, and an EPR4 (1+D-D.sup.2 -D.sup.3) channel. In these systems Viterbi detectors are frequently employed to achieve maximum likelihood detection of user data being played back from the recording medium. A modulation code for a PRML data recording and playback channel is selected to balance efficiency against robustness against errors.
The relevant technical literature provides examples of rate 8/9 (d-0, G=4/I=4) modulation codes, see e.g. U.S. Pat. Nos. 4,707,681 and 5,260,703. These references describe the utility and advantages of rate 8/9 modulation codes within the field of magnetic recording. While rate 8/9 modulation codes have been successfully employed within magnetic recording channels, a rate 16/17 (=0.941) modulation code achieves an approximately six percent (6%) increase in recording density over a standard rate 8/9 modulation code. The increased rate realized with a rate 16/17 modulation code means that each code bit recorded on the magnetic recording medium (disk or tape) contains approximately 6% more user data information than contained within a similar code bit in accordance with a rate 8/9 modulation code.
It has been proposed to map 16-bit user data words into a modulation code comprising 17;-bit code words, with the following specified coding constraints: the minimum zero run length distance d (or minimum number of zero cells between one cells within the code word), the maximum run length of an uninterrupted string of zeros G within the code word and accross codeword boundaries, and the maximum run length of an uninterrupted string of zeros I within a sequence of all-odd or all-even digit positions within the code. Thus, in a prior rate 16/17 modulation code, d=0, G=6 and I=6, meaning that the minimum zero run length was zero, the maximum run length of an uninterrupted string of zeros within the code word was six, and the maximum run length on an uninterrupted string of zeros within either an odd or even interleave was six. However, in this prior rate 16/17 modulation code, a large number of code words were required to be mapped, e.g. 65,536 words, as compared to 256 words required in the case of a rate 8/9 code. See "Rate 16/17 (0,6/6) Code", IBM Tech. Discl. Bull. Vol. 31, No. 8, Jan. 1989, pp. 21-23. While the approach reported in this article calls for symmetry in the code constraints with respect to bit positions in the code words and calls for insertion of a center bit between the code bytes, a number of drawbacks are present. In this prior approach, the assignment of the 65,536 code words was carried out using gated partitions, each of which was built upon a specific code structure. Also, a number of errors appear to be present in the disclosure, leading to difficulties with implementation, and uncertainty in achieving any successful utilization of the teaching thereof. Further, the described (0,6/6) code includes certain unesirable patterns, such as `11111 . . . 1`, which are avoided by the present invention.
In recording systems using partial response signals (e.g. PR4 or EPR4) and maximum likelihood (i.e. Viterbi algorithm) detection, it is critical that consecutive even and odd samples into the detector have nonzero values frequently. Additionally, it is critical to constrain consecutive samples leading into a partial response detector so that they frequently comprise nonzero values. This constraint ensures that timing recovery and automatic gain loops will have sufficient information to perform adequately.
As explained, while rate 16/17 modulation codes have been proposed for magnetic recording channels, the prior art approaches have been complicated, and problematic, both in theory and practice. A hitherto unsolved need has remained for an improved rate 16/17 modulation code which achieves higher magnetic recording densities without sacrificing reliability of detected data and without requiring undue circuit complexity.